Autodepositing composition containing vinylidene chloride based resin

ABSTRACT

Improved autodeposited coatings on metallic surfaces are formed by contacting the metallic surfaces with an autodepositing composition comprising an acidic aqueous coating solution containing dispersed solid resin particles of either an internally stabilized or externally stabilized vinylidene chloride polymer, an exemplary internally stabilized polymer being prepared from vinylidene chloride and a monomeric surfactant which includes an inorganic ionizable group, and an exemplary externally stabilized polymer being prepared from vinylidene chloride and a reactive comonomer, the resulting resin particles having surfactant adsorbed thereon, said autodeposited coatings having extremely good corrosion resistant properties without treatment with a reaction rinse such as a chromium rinse.

"This is a continuation of copending application(s) serial number07/627,897 filed on Dec. 13, 1990, now abandoned" which is acontinuation of copending application(s) serial number 07/178,625 filedon Apr. 7, 1988 (now abandoned), which is a continuation of applicationSerial No. 06/723,677, filed Apr. 16, 1985 (now abandoned), which is acontinuation-in-part of application Serial No. 06/629,911, filed Jul.11, 1984 (now abandoned), which is a continuation-in-part of applicationSerial no. 06/517,133, filed Jul. 25, 1983 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention -

This invention relates to the formation of resinous coatings on metallicsurfaces. More specifically, this invention relates to the deposition ofimproved resinous coatings without need for reaction rinses on metallicsurfaces by contacting the metallic surfaces with an acidic aqueouscoating solution containing dispersed solid resin particles.

Autodeposition involves the use of an aqueous resinous coatingcomposition of low solids concentration (usually less than about 10%) toform a coating of high solids concentration (usually greater than about10%) on a metallic surface immersed therein, with the coating increasingin thickness or weight the longer the time the metallic surface isimmersed in the composition. Autodeposition is similar toelectrodeposition, but does not require the aid of external electricalcurrent to cause the resin particles to deposit on the metal surface. Ingeneral, autodepositing compositions are aqueous acid solutions havingsolid resin particles dispersed therein.

2. Statement of the Related Art

Acidic aqueous coating solutions having dispersed therein solid resinparticles and having the capability of forming on metallic surfacesimmersed therein resinous coatings which grow with time are disclosed invarious patents.

Autodeposited coatings are disclosed, for example, in U.S. Pat. Nos.3,585,084; 3,592,699; and 4,373,050, each to Hall and Steinbrecher. Theautodepositing compositions disclosed in these patents are aqueoussolutions of acid and oxidizing agent, with solid resin particlesdispersed therein, particularly a latex combined with hydrofluoric acidand hydrogen peroxide. U.S. Pat. No. 3,709,743 discloses a similarcomposition comprising an acidic aqueous solution of dispersed resinsolids in which the acid component is nitric acid. U.S. Pat. Nos.4,347,172 and 4,411,937 disclose an improved autodepositing compositioncomprising hydrofluoric acid, ferric iron, for example, ferric fluoride,and dispersed resin solids. In this process, an oxidizing agent such asperoxide is disclosed as an optional ingredient.

In accordance with the disclosures of the aforementioned patents,autodeposited coatings of the type therein are treated with a solutionof chromium compounds prior to curing in order to impart to the coatingscorrosion resistant properties which are of an acceptable nature. Suchchrome treatments are disclosed in U.S. Pat. Nos. 3,647,567; 3,795,546;and 4,030,945.

Autodeposited coatings with better corrosion resistant properties (up toabout 1000 hours of salt spray resistance) which are achieved with achrome after treatment are disclosed in U.S. Pat. No. 4,313,861. Thispatent discloses that such improved autodeposited coatings are based onthe use in autodepositing compositions of particular acrylic copolymersas the resin component. The acrylic copolymers have a Tg of 2° to 50° C.and are prepared from either methacrylic acid or acrylic acid and atleast one other polymerizable ethylenically monounsaturated monomer, andoptionally, hydroxyl-containing monomer and/or a vinylenicallypolyunsaturated monomer.

It has also been recognized that the effectiveness of autodepositingcompositions is influenced by surfactant and gegenion concentrations andthat the corrosion resistance of autodeposited coatings can be improvedby the use of particular resin dispersions. For example, U.S. Pat. No.4,191,676 discloses an autodepositing composition containing adispersion of polymer particles in which the surfactant concentration inthe aqueous phase is below the critical micelle concentration. Aparticular class of resins for use in such compositions is prepared bypolymerizing: (i) 25 to 70 wt. % of a conjugated diene, for example,butadiene; (ii) 5 to 70 wt. % of a CH₂ =CHR compound in which R is anaryl or a cyano group, for example, styrene and acrylonitrile; (iii) 1to 50 wt. % of a vinylidene halide, for example, vinylidene chloride;and (iv) a monoethylenically unsaturated monomer having a functionalgroup, for example, acrylic acid and methacrylic acid. Autodepositedcoatings formed from compositions within the scope of the disclosure ofthis patent and treated with a chromium-containing solution exhibit goodcorrosion resistance after 240 hours of exposure in standard salt spraytests. U.S. Pat. No. 4,180,603 discloses a coating compositioncontaining epoxy resin solids and a cross-linking resin which when usedwithout a chrome after-treatment produces coating capable ofwithstanding up to 336 hours of exposure in standard salt spray tests.

Additional, U.S. Pat. Nos. which relate to autodeposition coatinginclude: 3,063,877; 3,776,848; 3,791,431; 3,936,546; 4,108,817;4,177,180; 4,186,219; and 4,318,944. The foregoing additional patentsdisclose the use of various polymer latices in autodeposition baths,including acrylates and styrene copolymers.

British patents 1,538,911 and 1,559,118 disclose the use of laticescontaining up to 55% by weight of vinylidene chloride monomer.

European published patent application 71,355 also disclosesautodeposition bath latices containing 70-95% vinylidene chloride and atleast one other monomer. The other monomers include: lower alkylacrylates and methacrylates; their derivatives; alkyl nitrile; vinylchloride; acrylamide; acrylamide derivatives; vinylsulfonic acid, itssalts, and esters; acrylic acid; methacrylic acid; and itaconic acid.However, this publication contains no information regarding the quantityof emulsifier to be used, and there is therefore no teaching of theinternal or external stabilization of the resulting vinylidene chloridecopolymer. The quantities of emulsifier normally employed in the art aredescribed in the Encyclopedia of Polymer Science and Technology, J.Wiley and Sons, Inc. (pub.), New York (1971) at 14:551,553. 1.01%emulsifier is suggested for a latex with 35% solids, and 1.48%emulsifier is suggested for a latex with over 50% solids. H. Barth inMethoden der Organischen Chemie, Houben-Weyl, Thieme Verlag (pub.),Stuttgart, Germany (1961) at 14/1:900 specifies an emulsifier quantityof 0.74%. In none of the above instances, is an internally or externallystabilized vinylidene chloride copolymer produced.

SUMMARY OF THE INVENTION

From the present state of the art, as described above, it is evidentthat there is a need for coating compositions and coating processescapable of producing resinous coatings having a relatively high degreeof corrosion resistance which is achieved without a chromeafter-treatment, chrome solutions being regarded as an industrial hazardand requiring costly waste treatment. The present invention is directedto an improvement in autodeposited coatings which are formed fromresin-containing coating compositions capable of forming continuousfilms having low moisture and oxygen permeability and which have greatercorrosion resistance than has been previously obtainable without the useof a chrome after-treatment. The advantages of this invention includethe following.

This invention provides improved autodeposited coatings having corrosionresistant properties of a much higher level than those possessed byautodeposited coatings heretofore known.

This invention further provides autodeposited coatings which can becured at low temperatures, for example, in the range of about 20° toabout 120° C. to yield coatings with physical and chemical propertieswhich are superior to prior art autodeposited coatings.

This invention also provides an autodepositing composition capable offorming an autodeposited coating which does not require the use ofchrome after-treatment, particularly, treatment with hexavalent chromiumor mixtures of hexavalent and trivalent chromium in aqueous rinsesolutions, to achieve a higher order of corrosion resistance than hasbeen possible with the use of prior art autodepositing compositions. Theachievement of such a goal would completely eliminate any possiblehealth hazards which might result from the use of suchchromium-containing solutions.

This invention still further provides autodeposited coatings withhardness of a higher order than present in autodeposited coatingsheretofore known.

This invention also provides autodeposited coatings with superiorresistance to solvents, particularly those solvents which frequentlycause damage to organic coatings used in the automotive industry.

This invention provides autodepositing compositions which can be appliedto metal and which can then be spray rinsed at relatively high pressureswithout loss of integrity of the freshly applied coatings, that is,coatings which have not been cured or baked to augment their adhesiveand/or cohesive properties.

This invention further provides autodeposited coatings which can becured by immersing in hot water (for example, water at a temperature upto 100° C.) or by spraying with hot water or by steam treating.

Finally, this invention provides a tightly adherent autodepositedcoating which will withstand unusually long salt spray and water soaktesting.

The present invention comprises the use in an autodepositing compositionof dispersed resin solids prepared from vinylidene chloride.

One aspect of the present invention includes an autodepositingcomposition containing an internally stabilized vinylidene chloridecopolymer. The term "internally stabilized" means that the vinylidenechloride-containing polymer includes an ionizable group which is part ofthe chemical structure of the polymer, that is, a chemically bonded partof the polymer structure. Latexes of such internally stabilizedvinylidene chloride-containing polymers can be prepared utilizing littleor no surfactant. An example of such a latex, described in detail below,is characterized by having therein particles of resin which are preparedby copolymerizing (A) vinylidene chloride with (B) monomers selectedfrom the group consisting of methacrylic acid, methyl methacrylate,acrylonitrile, and vinyl chloride and (C) a water soluble ionic materialwhich includes an inorganic ionizable group, for example, such as ispresent in sodium sulfoethyl methacrylate.

Another aspect of the present invention includes an autodepositingcomposition comprising an activating system of hydrofluoric acid andferric iron and containing a vinylidene chloride copolymer stabilizedwith an external surfactant, such copolymer containing at least about 50wt. % vinylidene chloride. In a preferred form of this aspect of theinvention, the autodepositing composition is prepared from a latex whichcontains such copolymer in the form of dispersed resin solids and inwhich the amount of surfactant is below the critical micelleconcentration.

Still another aspect of the present invention includes an autodepositingcomposition which includes, in the form of dispersed resin solids, acopolymer comprising at least about 50 wt. % of vinylidene chloride, thecomposition containing little or no surfactant in the aqueous phase.

The present invention includes also the use of autodepositingcompositions to form on metallic surfaces autodeposited coatings havingimproved properties, particularly improved corrosion properties. In themethod aspects of this invention, autodeposited coatings having improvedcorrosion resistance can be formed in the absence of a chromeafter-treatment or other type after-treatment designed to improve thecorrosion resistant properties of autodeposited coatings.

The present invention relates also to autodeposited coatings which arecharacterized by being essentially chromium-free, but having,nevertheless, a relatively high degree of corrosion resistance.

The preferred coating composition of the present invention is one inwhich the particles of resin, as described above are dispersed in anaqueous acidic solution which is prepared by combining hydrofluoric acidand a soluble ferric iron-containing ingredient, most preferably ferricfluoride.

Coating compositions within the scope of the present invention comprisethe use of a particular kind of resin or latex in combination with otheringredients which are effective in providing stable autodepositingcompositions that can produce hard, uniform, solvent resistant coatingson steel with an unusually high degree of corrosion resistance. Inaddition, coating compositions within the scope of the present inventionand containing a relatively small amount of resin solids, for example,about 3-8 wt. %, are effective in forming on a metallic surface immersedtherein a resinous coating which grows in thickness at a relatively fastrate, producing, for example, a coating having a thickness of as much as0.5 to 1 mil or more when the metallic surface is immersed in thecomposition for as short a time as about 90 seconds. As will be seenfrom examples set forth below, coating compositions within the scope ofthe present invention can be used to form coatings which have a highdegree of corrosion resistance. The ability of the coating compositionto coat rapidly has the important advantage of allowing the user toaccelerate production rates in that it is possible to produce coatingsof desired thicknesses within relatively short periods of time andcoatings which also have excellent corrosion resistant propertieswithout the need for a chrome treatment.

Another invention of the present invention is that autodepositedcoatings formed pursuant thereto can be cured efficiently by subjectingthem to water or steam for a relatively short period of time at arelatively low temperature.

Coating compositions within the scope of the present invention providecoatings with far superior corrosion resistance than could previously beobtained by the use of autodeposition. The corrosion resistance ofcoatings formed by the present process is so dramatically improved thatthe process may be used in applications which previously were reservedfor coating only by electrodeposition. Coating compositions usingvinylidene chloride copolymer in accordance with this invention alsopermit substantial savings by reducing the typical processing sequenceto four stages, and lowering the curing temperature, for example, to120° C., and lower.

Coatings produced in accordance with the present invention fromvinylidene chloride copolymers provide excellent hardness and scratchresistance as well as excellent appearance and solvent resistance.

DETAILED DESCRIPTION OF THE INVENTION

The particular resins, mentioned briefly above and described in detailbelow, are particularly well suited for use in autodepositing coatingcompositions and processes of the type described in U.S. Pat. No.4,191,676 noted above and as modified herein.

Speaking generally, the acidic aqueous coating compositions of theaforementioned type function to attack and dissolve from a metallicsurface contacted therewith metal ions in an amount sufficient todirectly or indirectly cause resin particles in the region of themetallic surface to deposit thereon in a continuous fashion, that is, ina manner such that there is a buildup in the amount of resin depositedon the surface the longer the time the surface is in contact with thecomposition. This deposition of the resin on the metallic surface isachieved through chemical action of the coating composition on themetallic surface. The use of electricity which is necessary for theoperation of electrocoating methods is not required.

Basic constituents of an autodepositing composition are water, resinsolids dispersed in the aqueous medium of the composition and activator,that is, an ingredient(s) which converts the water/resin compositioninto one which will form on a metallic surface a resinous coating whichincreases in thickness or weight the longer the surface is immersed inthe composition. Various types of activators or activating systems areknown, for example, as reported in U.S. Pat. Nos.: 3,592,699; 3,709,743;4,103,049; 4,347,172; and 4,373,050, the disclosures of which areincorporated herein by reference. The activating system generallycomprises an acid/oxidizing system, for example: hydrogen peroxide andHF; HNO₃ ; and a ferric-containing compound and HF; and other solublemetal-containing compounds (for example, silver fluoride, ferrous oxide,cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate,chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide,and silver nitrate in an amount between about 0.025 and about 50 g/l)and an acid that can be used alone or in combination with hydrofluoricacid, and including, for example, sulfuric, hydrochloric, nitric, andphosphoric acid, and an oranic acid, including, for example, acetic,chloracetic, and trichloracetic.

The preferred activating system comprises a ferric-containing compoundand hydrofluoric acid. Thus, a preferred autodepositing compositioncomprises a soluble ferric-containing compound in an amount equivalentto about 0.025 to about 3.5 g/l ferric iron, most preferably about 0.3to about 1.6 g/l of ferric iron, and hydrofluoric acid in an amountsufficient to impart to the composition a pH within the range of about1.6 to about 5.0. Examples of the aforementioned ferric-containingcompounds are ferric nitrate, ferric chloride, ferric phosphate, ferricoxide, and ferric fluoride, the last mentioned being preferred.

U.S. Pat. Nos. 4,347,172 and 4,411,937 which disclose the aforementionedtype of preferred activating system disclose the optional use in thecomposition of an oxidizing agent in an amount to provide from about0.01 to about 0.2 oxidizing equivalent per liter of composition.Suitable oxidizing agents are those commonly known as depolarizers.Examples of oxidizing agents are hydrogen peroxide, dichromate,permanganate, nitrate, persulfate, perborate, p-benzoquinone andp-nitrophenol. Hydrogen peroxide is mentioned as preferred. Thepreferred composition for use in the present invention does not includethe use of an optional oxidizing agent as disclosed in theaforementioned '172 and '937 patents.

With respect to the resin constituent of the autodepositing composition,in accordance with the present invention, and as between the externallyand internally stabilized vinylidene chloride-containing resins, thepreferred class of resins for use in the present invention is theinternally stabilized class. In effect, internally stabilized polymersor resins include as part of their chemical structure a surfactant groupwhich functions to maintain polymer particles or resin solids in adispersed state in an aqueous medium, this being the function alsoperformed by an "external surfactant", that is, by a material which hassurface-active properties and which is adsorbed on the surface of resinsolids, such as those in colloidal dispersion. As is known, the presenceof an external surfactant tends to increase the water sensitivity ofcoatings formed from aqueous resin dispersions containing the same andto adversely affect desired properties of the coatings. The presence ofundue amounts of surfactant in autodepositing compositions can lead toproblems, as described in U.S. Pat. No. 4,191,676, the disclosure ofwhich is incorporated herein by reference, particularly as regards itsdescription respecting surfactants and amounts thereof in autodepositingcompositions. As discussed in this patent, the presence of an undueamount of surfactant in autodepositing compositions can deter thebuild-up of resin particles on the metallic surface being coated. Inaddition, the presence of undue amounts of surfactant can also adverselyaffect desired coating properties, for example, corrosion resistantproperties. An advantage of internally stabilized vinylidenechloride-containing polymers is that stable aqueous dispersions,including acidic aqueous dispersions of the type comprisingautodepositing compositions, can be prepared without utilizing externalsurfactants. (It is noted that there is a tendency in the literature touse interchangeably the following terms in connection with describingsurface active materials which are used in polymerization processes forpreparing polymers of the type to which the present invention relates:surfactant, wetting agent, emulsifier or emulsifying agent anddispersing agent. As used herein, the term "surfactant" is intended tobe synonymous with the aforementioned.) Various types of internallystabilized vinylidene chloride-containing polymers are known and speciesthereof are available commercially. In accordance with the presentinvention, they can be used to excellent advantage in realizingimportant improvements in the field of autodeposition.

Various surfactants which function to maintain polymeric particles indispersed state in aqueous medium include organic compounds whichcontain ionizable groups in which the anionic group is bound to theprincipal organic moiety of the compound, with the cationic group beinga constituent such as, for example, hydrogen, an alkali metal, andammonium. Speaking generally, exemplary anionic groups of widely usedsurfactants contain sulfur or phosphorous, for example, in the form ofsulfates, thiosulfates, sulfonates, sulfinates, sulfaminates,phosphates, pyrophosphates and phosphonates. Such surfactants compriseinorganic ionizable groups linked to an organic moiety.

Although various ways may be used to introduce into the molecularstructure of the vinylidene chloride resin such ionizable groups, it isbelieved that the most widely used method for preparing such resins willinvolve reacting vinylidene chloride with a monomeric surfactant andoptionally one or more other monomers. In such reaction, the monomericsurfactant comprises a material which is polymerizable with monomericvinylidene chloride or with a monomeric material which is polymerizablewith monomeric vinylidene chloride and which is ionizable in thereaction mixture and in the acidic aqueous medium comprisingautodepositing compositions.

With respect to particular resins that can be used in the coatingcomposition of the present invention, a preferred class can be preparedby copolymerizing (A) vinylidene chloride monomer with (B) monomers suchas methacrylic acid, methyl methacrylate, acrylonitrile, and vinylchloride and (C) a water soluble ionic material such as sodiumsulfoethyl methacrylate. Although the constituents comprising theabove-desired resin can vary over a relatively wide range, in generalthe resin will comprise the polymerized constituents in the followingamounts:

1) between 45 and about 99 weight percent based on the total weight ofmonomers used of vinylidene chloride monomer;

2) from about 0.5 to 30 weight percent based on the total weight of (1)and (2) of a second relatively more hydrophilic ethylenicallyunsaturated monomeric material wherein such monomeric material has asolubility in both the water phase and the oil phase of the polymerlatex of at least 1 weight percent at the temperature of polymerization;and

3) from about 0.1 to about 5 weight percent based on the total weight ofother monomers of an ionic significantly water-soluble material which iscopolymerizable with (2) and is selected from the group of sulfonicacids and their salts having the formula:

    R--Z--Q--(SO.sub.3).sup.- M.sup.+

Examples of resins prepared from such monomers are disclosed in U.S.Pat. No. 3,617,368. As disclosed in this patent, the radical "R" isselected from the group consisting of vinyl and substituted vinyl, forexample, alkyl-substituted vinyl; the symbol "Z" represents adifunctional linking group which will activate the double bond in thevinyl group; --Q-- is a divalent hydrocarbon having its valence bonds ondifferent carbon atoms; and the symbol "M⁺ " represents a cation.

The relatively hydrophilic monomers of (2) above include those materialswhich are readily copolymerizable with (1) in aqueous dispersion, thatis, which copolymerize within a period of about 40 hours at atemperature ranging from the freezing point of the monomeric serum up toabout 100° C., and which have a solubility in both the water and the oilphase of the polymer latex of at least 1 weight percent at thetemperature of polymerization. Exemplary of preferred materials,particularly when used in conjunction with monomeric vinylidene chlorideare: methacrylic acid and methyl methacrylate. Other monomers which maybe advantageously employed include the hydroxyethyl and propylacrylates, hydroxyethylmethacrylate, ethyl hexylacrylate, acrylic acid,acrylonitrile, methacrylonitrile, acrylamide, and the lower alkyl anddiallkylacrylamides, acrolein, methylvinyl ketone, and vinyl acetate.

These monomers, which can be employed in amounts of from 0.5 to 30weight percent, based on the total weight of the nonionic monomers used,provide for the necessary reactivity with the copolymerizable ionicmaterial of (3) and also provide for the required water solubility ofthe interpolymer in water. Thus, such materials may be referred to as"go-between" monomers. It is to be understood that the optimum amount ofsuch relatively hydrophilic monomers may vary somewhat within theprescribed range depending upon the amount of hydrophobic monomer usedin preparing the resin, as well as upon the amount and type of thecopolymerizable ionic monomer used.

The copolymerizable ionic monomers used in preparing the aforementionedtype resins are those monomeric materials which contain in theirstructure both an ionizable group and a reactive double bond, aresignificantly soluble in water, are copolymerizable with the hydrophilicmonomer constituent (2) and in which the substituent on the double bondis chemically stable under the conditions normally encountered inemulsion polymerization. Examples of the difunctional linking group (Z)which will activate the double bond present in the vinyl group includegroups of the structure: ##STR1## and the like. The alkyl group ispreferably alkyl of 1 to 8 carbon atoms, especially methyl, ethyl orpropyl. Examples of the aformentioned divalent hydrocarbon having itsvalence bonds on different carbon atoms include alkylene and arylenedivalent hydrocarbon radicals. Although the alkylene (CH₂) group cancontain up to about 20 carbon atoms, it will generally have 1 to about 8carbon atoms.

The solubility of the defined copolymerizable ionic material asdescribed herein is strongly influenced by the cation M⁺. Exemplarycations are the free acids, alkali metal salts, ammonium and amine saltsand sulfonium and quaternary ammonium salts. Preferred are the freeacids, alkali metal salts, particularly sodium and potassium, andammonium salts.

It is further noted that, with one of the ions above, and the usualchoices for R and Z, the solubility of the monomer depends on Q. Asindicated, this group can be either aliphatic or aromatic and its sizewill determine the hydrophilic/hydrophobic balance in the molecule, thatis, if Q is relatively small, the monomer is water soluble, but as Qbecomes progressively larger the surface activity of such monomerincreases until it becomes a soap and ultimately a water insoluble wax.It is to be understood, however, that the limiting size of Q depends onR, Z, and M⁺. As exemplary of the above, it has been found that sodiumsulfoalkyl methacrylate of the formula: ##STR2## wherein n is 2, is ahighly acceptable copolymerizable ionic material for use in the presentinvention.

Further, the selection of R and Z is governed by the reactivity neededand the selection of Q is usually determined by the reaction used toattach the sulfonic acid to the base monomer (or vice versa).

Processes for preparing latexes containing resins of the aforementionedtype are known, such latexes being commercially available and beingreferred to herein as "self-stabilizing latexes", that is, latexes, thepolymeric particles of which contain in the polymer molecule functionalgroups that are effective in maintaining the polymeric particlesdispersed in the aqueous phase of the latex. As mentioned above, suchlatexes do not require the presence of an external surfactant tomaintain the particles in their dispersed state. Latexes of this typegenerally have a surface tension very close to that of water (about 72dynes/cm). It has been observed that autodepositing compositionscontaining such latexes form coatings which build up at a relativelyfast rate.

An exemplary method for preparing such latexes involves preparation ofan aqueous dispersion by an essentially continuous, carefully controlledaddition of the requisite polymerization constituents (includingpolymerization initiator systems, if desired) to the aqueous medium. Insuch process, it is often preferred to first add a small amount of themonomeric materials to the aqueous medium having the desired pH value,followed by the subsequent addition of the necessary polymerizationinitiator, to form a polymeric seed latex in order to aid in the controlof particle size. When forming such polymeric seed latexes, very smallamounts of conventional surfactants, such as alkali soaps or the like,may be incorporated in the aqueous medium to further aid in theattainment of particles of desired size. The addition of suchsurfactants, however, is not critical for the production of the highlystable, internally stabilized, aqueous colloidal dispersions ofpolymeric particles of the type described above. In any event, additionsof surfactants are limited so that the total amount present in theaqueous phase of the final coating solution is less than the criticalmicelle concentration, as taught in aforementioned U.S. Pat. No.4,191,676. Following the formation of the polymeric seed latex, theremaining polymerization constituents are simultaneously andcontinuously added under carefully controlled conditions to the aqueousmedium.

Highly stable polymer latexes for use in the present invention arecharacterized by the virtual absence of undesirable coagulum which oftenresults when polymeric latexes are stabilized by conventional watersoluble surfactants. Thus, such latexes combine the highly beneficialproperties of optimum colloidal stability, reduced viscosities atrelatively high polymer solids content, low foaming tendencies andexcellent product uniformity and reproducibility. Such highly stablelatexes which are internally stabilized are disclosed, for example, inaforementioned U.S. Pat. No. 3,617,368.

A preferred embodiment of this invention comprises the use of vinylidenechloride-containing latexes in which a water soluble ionic material suchas, for example, sodium sulfoethyl methacrylate is copolymerized withthe comonomers comprising the copolymer. Sodium sulfoethyl methacrylateis particularly effective for use with monomeric vinylidene chloride andthe relatively hydrophilic monomers methyl methacrylate or methacrylicacid when used in the amounts and in the manner called for by thepresent invention.

Particularly preferred latexes for use in this invention are latexeswith about 35 to about 60 weight % solids comprising a polymericcomposition prepared by emulsion polymerization of vinylidene chloridewith one or more comonomers selected from the group consisting of vinylchloride, acrylic acid, a lower alkyl acrylate (such as methyl acrylate,ethyl acrylate, butyl acrylate), methacrylic acid, methyl methacrylate,acrylonitrile, methacrylonitrile, acrylamide, and methacrylamide andstabilized with sulfonic acid or sulfonic acid salt of the formula:

    R--Z--(CH.sub.2).sub.n --(SO.sub.3).sup.- M.sup.+

wherein R represents vinyl or lower alkyl-substituted vinyl, Zrepresents one of the functional groups: ##STR3## n is an integer from 1to 20 (preferably 1 to 6) and M⁺ is hydrogen or an alkali metal cation,preferably sodium or potassium.

A subgroup of preferred polymers are those having at least about 50% byweight of vinylidene chloride, but less than about 70%, and about 5 toabout 35% vinyl chloride, and about 5 to about 20% of a vinyl compoundselected from the group consisting of acrylic acid, methyl acrylate,ethyl acrylate, butyl acrylate, methacrylic acid, methyl methacrylate,acrylonitrile, methacrylonitrile, acrylamide and methacrylamide, andcombinations thereof, and about 1 to about 3% by weight of sulfoethylmethacrylate.

A particularly preferred group of latexes, however, are latexescontaining about 30 to about 70 weight % of solids formed by emulsionpolymerization of about 50 to about 99% vinylidene chloride based ontotal weight of polymer and about 0.1 to about 5% by weight ofsulfoethyl methacrylate, with optionally other comonomers selected fromthe group consisting of vinyl chloride, acrylic and methacrylic monomerssuch as acrylonitriles, acrylamides, methacrylamides and mixturesthereof in amounts between about 5 and about 50% by weight, andsubstantially free of unpolymerized surfactant or protective colloid.

Among other preferred subclasses of resin for use in this invention aredispersions of copolymers of about 50 to about 90% by weight vinylidenechloride, about 5 to about 30% by weight of butyl acrylate and about 1to about 2% by weight of sulfoethyl methacrylate based on the totalweight of polymer. Another preferred subclass of polymers are thelatexes of vinylidene chloride-containing polymers internally stabilizedwith sulfoethyl methacrylate and free of surfactant, and includingoptionally vinyl chloride and one or more acrylic comonomers.

Another preferred vinylidene chloride-containing copolymer is onecomprising about 15 to about 20 weight % vinyl chloride, about 2 toabout 5 weight % butyl acrylate, about 3 to about 10 weight %acrylonitrile, about 1 to about 2 weight % sulfoethyl methacrylate. Thisparticular copolymer will have less than 70% by weight vinylidenechloride copolymer based upon total weight of comonomers (including thesulfoethyl methacrylate) used in the emulsion polymerization.

In its most preferred form, the present invention comprises the use ofinternally stabilized vinylidene chloride-containing resins of the typeused in Example 1 reported hereinbelow. Such resins are of relativelyhigh crystallinity. Exemplary crystalline resins are described in U.S.Pat. No. 3,922,451 and aforementioned U.S. Pat. No. 3,617,368. Generallyspeaking, crystalline vinylidene chloride-containing resins comprise arelatively high proportion of vinylidene chloride, for example, at leastabout 80 wt. % thereof.

Although internally stabilized vinylidene chloride-containing resins arepreferred for use in the practice of the present invention, vinylidenechloride-containing resins stabilized with external surfactants can alsobe used. The use of such resins forms autodeposited coatings whichexhibit chemical and physical properties superior to those ofautodeposited coatings prepared from resins heretofore known in thestate of the art. The externally stabilized vinylidenechloride-containing resins contain a high proportion of vinylidenechloride, that is, at least about 50 wt. %, and preferably in excess of50 wt. %. The amount of vinylidene chloride comprising the resin shouldbe less than 100 wt. % for the reason that the pure homopolymer ofvinylidene chloride is thermally unstable. It undergoesdehydrochlorination, that is, an "unzippering" depolymerization whichleads to complete breakdown of the polymer. The proclivity for thermalinstability can be greatly reduced by copolymerizing the vinylidenechloride monomer with comonomers, for example, acrylic comonomers, whichcan be used to break up the length of the vinylidene chloride sequencesby inserting more thermally stable comonomers into the chain therebypreventing further unzippering. In simplistic terms, the use ofcomonomers in vinylidene chloride polymers is akin to jamming a zipperat a number of points which prevents it from opening more than a shortdistance.

Examples of monomers that can be copolymerized with vinylidene chlorideto form a thermally stable copolymer include one or more of vinylchloride, acrylic acid, methacrylic acid, methylacrylate, methylmethacrylate, ethylacrylate, butylacrylate, acrylonitrile,methacrylonitrile, acrylamide and methacrylamide. A few examples ofexternally stabilized vinylidene chloride polymers that can be used inthe practice of the present invention are the following: (A) about 50 toabout 90 wt. % vinylidene chloride, about 5 to about 20 wt. %acrylonitrile and about 5 to about 20 wt. % butyl acrylate; (B) about 60to about 76 wt. % vinylidene chloride, about 4 to 10 wt. % ethylhexylacrylate or methacrylate, and about 1 to about 4 wt. % acrylic acid; and(C) 66 wt. % vinylidene chloride and 34 wt. % vinyl chloride. Inpreferred form, the externally stabilized vinylidene chloride-containingresin is crystalline in nature.

Externally stabilized resins of the above type are prepared typically byemulsion polymerization utilizing a sufficient amount of surfactant tomaintain the resulting resin particles in a dispersed state in theaqueous medium of the reaction mixture. The nature of this resin type isthat the structure of the polymer molecule requires that surfactant bepresent to maintain the colloidal dispersion of the resulting latex, thesurfactant being adsorbed on the surface of the resin particles.

Examples of surfactants (emulsifiers) that can be used to prepare suchlatexes are: sodium dodecylbenzene sulfonate, alkyl sulfates, sodiumdioctyl sulfosuccinate, alkylphenolicethoxylate sulfonates, sodiumdodecyldiphenyl oxide disulfonate, sodium oleoyl isopropanolamidesulfosuccinate, and sodium lauryl sulfate. These surfactants oremulsifiers are exemplary only; accordingly, it should be understoodthat the practice of this invention is not limited to latexes containingthe aforementioned surfactants. For example, there can be used anyanionic surfactant which will lower the interfacial tension between themonomeric reactants and water sufficiently to result in the formation ofstable colloidal dispersions of the monomers in the water and, inaddition, is stable in autodepositing compositions formulated therefrom.It is noted further that the activating system (for example, acid andoxidizer) of the autodepositing composition functions to dissolve fromthe metallic substrate positively charged cations which cause thenegatively charged latex polymer particles to autodeposit on themetallic surface. The anionic surfactant should be a material whichfunctions accordingly. Mixtures of two or more suitable anionicsurfactants may be used.

For reasons mentioned above, and as explained in aforementioned U.S.Pat. No. 4,191,676, the surfactant concentration of externallystabilized latexes should be relatively low so that the aqueous phase ofthe autodepositing composition has a surfactant concentration below thecritical micelle concentration and preferably below the surfactantconcentration which corresponds to the inflection point on a graph ofsurface tension versus the logarithm of surfactant concentration in thecomposition, as referred to in the aforementioned '676 patent.Accordingly, it is preferred that the latex containing the externallystabilized resin be prepared by emulsion polymerization with a very lowconcentration of surfactant. In the present state of the art, this isbest achieved by seed polymerization or semicontinuous polymerization asopposed to batch emulsion polymerization. In the seed polymerizationprocess or semicontinuous polymerization, the amount of surfactant canbe limited by adding surfactant and monomer in such a manner that themonomer continues to polymerize with particles already present ratherthan forming new particles. This gives greater uniformity of particlesize distribution and also gives good control of the total amount ofsurfactant in the latex. The stability of the colloidal latex can bepartially attributed to charged polymer end groups whose provenance isthe polymerization initiator.

In general, such latexes will have a relatively high surface tension,that is, at least about 40 dynes/cm. Such latexes can be used in thepractice of the present invention, and preferably, there are used suchlatexes that have a surface tension of about 55 to 70 dynes/cm. Inparticular, such latexes in which there is no protective colloid arepreferred class for use in the present invention.

Polymers having a vinylidene chloride content of between about 50 wt. %and about 90 wt. % or higher, based upon the total weight of polymer,can be used. When vinyl chloride is employed as one of the co-monomers,the vinylidene chloride content can be less than about 70 wt. %. Thetotal chloride content, however, is preferably 50 wt. % or more based ontotal polymer weight. Generally speaking, the internally stabilizedlatexes which are the preferred class of latexes according to thisinvention can be used at even lower chloride contents, thereby enablingthe inclusion of other copolymers to enhance the desirable polymercharacteristics such as hardness, gloss, solvent resistance and thelike, in addition to corrosion resistance attributable to reduced vaporpermeability.

Latexes for use in the composition of the present invention areavailable commercially. Examples of such latexes are the Saran latexessuch as, for example, Saran 143 and Saran 112 available from DowChemical Co., the Serfene latexes available from Morton Chemical, andthe Haloflex latexes such as, for example, Haloflex 202 available fromImperial Chemicals Industries.

If desired, the autodepositing composition can be prepared from two ormore latexes containing different resins. Such a composition willcomprise a blend of the vinylidene chloride-containing polymersdescribed above, typically in an amount of about 50 to about 95% of thetotal resin solids, and one or more other latexes including, forexample, styrenebutadiene resins, poly(vinyl chlorides), acrylic resinsand the like.

The amount of the resin comprising the coating composition of thepresent invention can vary over a wide range. The lower concentrationlimit of the resin particles in the composition is dictated by theamount of resin needed to provide sufficient material to form a resinouscoating. The upper limit is dictated by the amount of resin particleswhich can be dispersed in the acidic aqueous composition. In general,the higher the amount of resin particles in the composition, the heavierthe coating formed, other factors being the same. Although coatingcompositions can be formulated with a range of about 5 to about 550 g/lof resin solids, the amount of the resin solids will tend to varydepending on the other ingredients comprising the composition and alsoon the specific latex or resin used. For many applications, good resultscan be achieved utilizing about 50 to about 100 g/l of resin solids inthe composition.

Optional ingredients can be added to our composition as desired. Forexample, it is believed that the composition of the present inventionwill be used most widely in application where it is desired to applypigmented coatings to the metallic substrate. For this purpose, suitablepigments can be included in the composition. Examples of pigments thatcan be used are carbon black, phthalocyanine blue, phthalocyanine green,quinacridone red, benzidene yellow and titanium dioxide. The pigmentshould be added to the composition in an amount which imparts to thecoating the desired color and/or the desired depth or degree of hue. Itshould be understood that the specific amount used will be governed bythe specific pigment used and the color of coating desired. Excellentresults have been achieved by using an aqueous dispersion in an amountsuch that the composition contains about 0.2 to about 3 g of furnaceblack/100 g of resin solids.

Many pigments are available in aqueous dispersions which may includesurfactants or dispersing agents for maintaining the pigment particlesin dispersed state. When utilizing such pigment dispersions in thecomposition of the present invention, they should be selected so thatthe surfactant concentration in the aqueous phase of the composition isbelow the CMC, preferably below the surfactant concentration whichcorresponds to the inflection point on a graph of surface tension versusthe logarithm of surfactant concentration in the composition. Thesurfactant should preferably be selected from those indicated above withrespect to the preparation of the externally stabilized latexes.Suitable pigmented compositions are illustrated in examples herein.

Colored coatings can be produced also by the use of dyes, examples ofwhich include rhodamine derived dyes, methyl violet, safranine,anthraquinone derived dyes, nigrosine, and alizarin cyanine green. Theseare but a few examples of dyes that can be used.

Examples of other additives that may be used in the autodepositingcomposition are those generally known to be used in formulating paintcompositions, for example, UV stabilizers, viscosity modifiers, etc.

If a surfactant is added to the composition, either as a component ofthe latex, or with a pigment dispersion, or with other ingredients oradditives, the total amount of surfactant in the aqueous phase of thecomposition should be maintained below the CMC. Preferably, the aqueousphase of the composition contains little or no surfactant.

In case a surfactant is utilized, the preferred surfactants are theanionic surfactants. Examples of suitable anionic surfactants are thealkyl, alkyl/aryl or naphthalene sulfonates, for example, sodiumdioctylsulfosuccinate and sodium dodecylbenzene sulfonate.

In preparing the autodepositing composition of the present invention,the constituents thereof can be admixed in any suitable way, forexample, as described in aforementioned U.S. Pat. No. 4,191,676. Inpreparing a bath of pigmented coating composition for use on anindustrial scale, it is preferred that the bath be prepared by admixing:

A) an aqueous concentrate comprising about 350 to about 550 g/l of theaforementioned vinylidene chloride-containing resin particles and about10 to about 550 g/l of pigment; and

B) an aqueous concentrate prepared from about 0.4 to about 210 g/l of HFand a water soluble ferric-containing compound in an amount equivalentto about 1 to about 100 g/l of ferric iron.

The bath can be prepared by stirring water into concentrate (A) andthereafter admixing therewith the required amount of concentrate (B)with stirring to provide a homogenous composition.

Various steps of the overall coating process in which the autodepositingcomposition of the present invention is used can be like those of theprior art, except as noted below. For example, cleaning of the metallicsurface prior to coating and any water rinse steps effected subsequentto the cleaning step can be in accordance with the teachings ofaforementioned U.S. Pat. No. 4,191,676. With respect to contacting themetallic surface with the autodepositing composition, it is believedthat, for most applications, desired coating thicknesses can be obtainedby immersing the metallic surface in the composition for a period oftime within the range of about 30 seconds or even less to about 3minutes. Good results have been achieved utilizing a time of immersionof not more than about 90 to about 100 seconds with compositionscontaining about 5 to about 10 wt. % of resin solids. However, it shouldbe understood that longer or shorter periods of time can be used.Agitating the composition aids in maintaining it uniform and inimproving the uniformity of the coatings formed. Other factors heldconstant, heating of the composition will result in heavier coatings.However, satisfactory results can be obtained by operating the coatingprocess at ambient temperature.

As is illustrated in examples reported below, coating compositionswithin the scope of the present invention are effective in formingcoatings which upon being immediately withdrawn from the coatingcomposition are initially tightly adherent to the metallic substrate.For example, such coatings resist being removed from the substrate whenthey are rinsed by spraying water under pressure against the coatedsurface, as for example, is shown in examples below.

Water rinsing the coated surface after it has been withdrawn from thecomposition, and before significant drying takes place is effective inremoving therefrom residuals such as acid and other ingredients of thecomposition that adhere to the coated surface. If such residuals areallowed to remain on the coated surface, they may change or adverselyaffect the quality of the coating. For a specific application, adetermination can be made as to whether the residuals cause adverseeffects which are not tolerable. If they do, they should be removed, forexample, by water rinsing with tap or deionized water. Further, theresiduals can be removed or rendered unreactive by treatment with analkaline solution suitably a mild alkaline solution, for example, asolution of about 0.1 to about 2 g/l of caustic. If removal of residualsis not necessary to the finish required, this step of removing them canbe avoided.

Upon partially or completely air drying or baking the coating, thesuperficial layer of unreacted coating composition adheres to theunderlying and initially adherent coating in a manner such that it iscapable of withstanding water rinsing. Resinous coatings formed inaccordance with the present invention can be cured by air drying at roomtemperature for sufficient time, though an elevated temperature isdesirable to effect a thorough cure in practical times.

Following any rinse steps employed after the coated surface is withdrawnfrom the composition, the coating should be cured. Fusion of theresinous coating renders it continuous, thereby improving its resistanceto corrosion and its adherence to the underlying metallic surface.

The conditions under which the curing and/or fusion operation is carriedout depend somewhat on the specific resin employed. In general, it isdesirable to apply heat to fuse the resin although, as noted above,resins of the type employed in this invention can be cured at roomtemperature. Generally the corrosion resistant, hardness, and solventresistant properties of coatings fused at elevated temperatures havebeen observed to be better than coatings which have been air dried.However, there are applications where air dried coatings can be usedsatisfactorily. The fusion of the coating should be carried out undertemperature and time conditions which do not adversely affect thedesired properties of the coating. Exemplary conditions used in fusingcoatings produced according to the present invention are temperatureswithin the range of about 20° C. to 120° C. for periods of time withinthe range of about 10 to about 30 minutes, depending on the mass of thecoated part. Baking the coating for a period of time until the metallicsurface has reached the temperature of the heated environment has beenused effectively.

When baked in an oven, the coating reaches the proper "curing" orheating temperature for the full development of coating properties whenthe metal part reaches that temperature. For this reason, parts that areconstructed of thicker steel require longer times to reach the requiredtemperature. For massive parts, it may not be possible to reach therequired temperature without deleteriously affecting the coating andcausing it to degrade.

In some cases, it is possible to overcome this problem by resorting toinfrared radiation curing. In this case, it is possible to cure thecoating without simultaneously raising the temperature of the metal tothe required temperature. However, infrared radiation curing ispracticable only for simple geometric shapes since the area to be curedmust be exposed to the infrared. In using infrared radiation curing, allcoated surfaces must be visible to the infrared source, that is, theentire coated surface must "see" the infrared.

Autodeposited coatings of the type described above, can be "cured" orheated to such a degree that those properties which depend upon properbake schedule, such as, for example, corrosion resistance, adhesion, andhardness can be readily achieved, even on massive parts, as well asrelatively thin wall parts, by the simple expedient of treating thefreshly coated part with hot water or steam. Exemplary treating meansinclude spraying the freshly applied coating with hot water, immersingthe freshly coated part in hot water and exposing the freshly depositedcoating to an atmosphere of steam. Utilizing the present invention, heatcan be transferred more quickly into the coated mass with the resultthat the temperatures needed for full development of coating propertiesare reached more quickly than they are reached when heating the coatedpart in air.

The curing of autodeposited coatings in general, including autodepositedcoatings of the type described above, is the subject of U.S. Pat.application, Ser. No. 06/629,924 filed Jul. 11, 1984 in the name ofBashir M. Ahmed, and entitled "Water or Steam Cure of AutodepositedCoatings". The disclosure of said application, as it pertains to theautodeposited coatings described therein and the water or steam curingthereof, is incorporated herein by reference. As disclosed in saidapplication, the types of autodeposited coatings that particularly lendthemselves to being so cured are those comprising resins which developfully their coating properties at the elevated temperatures used, thatis, temperatures up to 212° F. in the case of water and steam atatmospheric pressure, and higher temperatures in the case of superheated steam, and which are not degraded at such temperatures. As alsodisclosed in said application, it is believed that such a curing processwill be used most widely in curing autodeposited coatings comprisingresins which have a minimum film temperature (MFT) or glass transitiontemperature (Tg) of no greater than about 140° F., prefereably nogreater than about 100° F., and most preferably no greater than about85° F. Vinylidene chloride-containing resins within the scope of thepresent invention can have Tg's within the range of about 30° F. toabout 85° F. They can also have an accelerated cure temperature of nogreater than about 300° F. With respect to resins having a Tg well belowroom temperature, improvements can be realized by curing with waterhaving a temperature of at least 70° F.

Resins which do not inherently have Tg's or MFT's in the desired rangecan be modified by the use of solvents or plasticizers in accordancewith the state of the art to reduce their Tg and MFT valuesappropriately.

The temperature and time of treatment of the autodeposited coating willdepend upon the nature of the particular resin comprising the coating.The treating conditions should be selected so that the properties of thecoating are fully developed and so that the coating is not affectedadversely. Exemplary conditions include treating times of about 5seconds to about 5 minutes (although longer times can be used) attemperatures within the range of about 185° F. to about 212° F.(although higher temperatures can be used in the case of super heatedsteam). It is believed that many applications will require no more thanabout 2 to 3 minutes and even less time of pretreatment when usingtemperatures within the aforementioned range. Particularly, in the caseof steam, there may be many applications in which the treating time isless than 5 seconds. Autodeposited coatings comprising vinylidenechloride-containing resins of the type described above can be properlyheated or "cured" to achieve full development of coating properties byimmersion of the freshly formed autodeposited coating into hot water foras little as about 5 to about 30 seconds at temperatures of about 185°F. to about 212° F.

Steam curing has a number of advantages over the "hot water cure" methoddescribed above. One advantage accruing to the use of a steam atmospherefor curing the freshly formed coating is that the parts need not beimmersed. This is important when parts are being processed on a conveyerline. In order to immerse a part in hot water, the conveyor line mustchange directions as the part is carried down into a tank of hot water.Once the part has been "cured", the convenyor must then changedirections again to remove the part from the hot water tank. By the useof steam to effect curing of the autodeposited coating, the conveyorsimply carries the parts into a tunnel which contains the steam. Thereis no need for the convenyor to change directions as in an immersionprocess.

A further advantage accruing to the use of steam for curingautodeposited coatings is that steam has a higher calorific value thandoes hot water. For example, a gram of steam at 212° F. has a higherheat content than a gram of water at the same temperature. This providesa greater energy source for transferring energy rapidly to the freshlyformed autodeposited coating. Still another advantage accruing to theuse of steam is that only a relatively small amount of water need beconverted to steam as opposed to raising the temperature of an entiretank of water to the operating temperature.

The steps of rinsing the freshly formed autodeposited coating to removetherefrom residuals and curing of the coating can be combined into onestep. Thus, for example, rinsing and curing can be done simultaneouslyby spraying with hot water or immersing the freshly formed autodepositedcoated surface in a water bath. In addition, the water or steam cureprocess can be used in combination with heretofore known curing methods.For example, a short treating time in accordance with the curing methodof the present invention can be used to quickly heat the coating (whichsurprisingly can result in drying of the coating) followed by baking.

An important characteristic of the vinylidene chloride-containingcoatings of the present invention is that they exhibit extraordinarycorrosion resistance without the use of treatments which are designed toincrease the corrosion resistance of autodeposited coatings. Examples ofsuch treatments include rinsing the freshly applied coating with aqueoussolutions of chromium compounds or with aqueous solutions of phosphoricacid. For example, as previously noted, U.S. Pat. Nos. 3,795,546 and4,030,945 disclose methods of treating freshly formed autodepositedcoatings with aqueous rinse solutions containing hexavalent chromium oraqueous solutions containing mixtures of hexavalent chromium andformaldehyde-reduced forms of hexavalent chromium to improve thecorrosion resistance of the autodeposited coatings. U.S. Pat. No.3,647,567 discloses the use of chromium-containing solutions and alsothe use of an aqueous solution of phosphoric acid. Although uncuredcoatings formed in accordance with the present invention can be treatedwith compositions designed to improve the corrosion resistance of curedautodeposited coatings, cured coatings within the scope of the presentinvention possess unusually high corrosion resistant properties withoutbeing so treated.

It is generally believed that corrosion of coated metal surfaces occurswhen moisture permeates the protective coating and permits the transportof electrons or ions between microcathode and microanode sites on themetal substrate through the electrolyte. Moreover, the water content ofthe protective coating significantly affects the adhesion of the coatingand, at high humidities, may actually cause the coating to separate fromthe metal. By decreasing the permeability of the protective coating tomoisture, therefore, ionic dissolution, ionic transport and diffusion,osmotic blistering, and losses of adhesion on exposure to high humidityare significantly mitigated.

Polymeric films exhibit decreasing moisture vapor transmission rate(MVTR) as their content of chemically bound chlorine increases. Purehomopolymers of vinylidene chloride, for example, comprise twochemically bound chlorine atoms for each monomer unit or more than 70wt. %. The MVTR decreases linearly with increasing content of chemicallybound chlorine so the homopolymer of vinylidene chloride has a very lowpermeability, on the order of 10 grams of moisture vapor through asquare meter of film 25 microns thick in 24 hours. By comparison, forexample, a latex film based on a polymer comprising a 1:1 ratio ofmethyl methacrylate and butyl acrylate has an MVTR of 1290 g/25 μ/m²/day.

The moisture vapor permeability and water sensitivity of vinylidenechloride copolymer films can be deleteriously affected by increasingconcentrations of surfactant used in the process for preparation of thevinylidene chloride-containing polymer. In an earlier part of thisdisclosure, it was shown that high surfactant concentrations are alsoundesirable in autodeposition. Accordingly, for purposes of thisinvention, it is preferred that the latex used in formulating thecomposition and the autodepositing composition itself contain a very lowconcentration of surfactant or no surfactant.

The vapor permeability, as measured by cast film water vaportransmission rate (WVTR), of preferred resins is less than about 50g/mil/m² /day and preferably less than 20 g/mil/m² /day. The film ofthese preferred resins, when applied in accordance with the preferredautodepositing method of his invention, that is, the method whichutilizes an autodepositing composition containing an hydrofluoricacid/ferric fluoride activating system, provides a coated surface inwhich the vapor permeability, based upon improved corrosion resistance,is substantially less than that of a film cast from the same latex.

Preferred operating steps for forming resinous coatings on steelsurfaces, for example, car frames made from hot rolled steel, whichcoatings provide excellent corrosion resistance after being subjected tosalt spray (ASTM B117) for at least 168 hours include the following:

A) cleaning the steel surface, preferably to the extent that awaterbreak-free film can be formed on the surface;

B) water rinsing the cleaned surface to remove there-from residualcleaning agent;

C) immersing the surface in the preferred pigmented coating composition,as described above, for a period of time of about 45 to about 90 secondsto form on the surface a coating having a thickness of about 0.4 toabout 1.2 mil;

D) withdrawing the coated surface from the composition and, eitherimmediately or after a partial air dry of about 30 to about 60 seconds,water rinsing the coated surface to removed therefrom residual coatingcomposition; and

E) drying the coated surface at a temperature within the range of about20° C. to about 120° C. for a period of time of about 10 to about 30minutes.

An alternative to Step (E) above is to immerse the coated part in hotwater or subject it to steam as described above.

In accordance with the present invention, there is provided anautodeposited coating comprising an internally stabilized vinylidenechloride-containing resin adhered to a metallic surface, said resincomprising a plurality of polymeric molecules having a plurality ofnegatively charged groups in chemically bonded form, including polymericmolecules of resin contiguous to said surface chemically bonded theretoby a plurality of said negatively charged groups, and polymericmolecules of resin comprising said coating chemically bonded togetherthrough said negatively charged groups, said groups being chemicallylinked by metal atoms, the source of which is said metallic surface,said coated surface being essentially free of chromium and havingcorrosion resistance properties characterized by less than about 1 mmloss of adhesion at the scribe when subjected to 5% neutral salt sprayat 95° F. ASTM B-117 for 500 hours or more. Such autodeposited coatingsdiffer from coatings formed from autodepositing compositions containingexternally stabilized resins. As pointed out in aforementioned U.S. Pat.No. 4,191,676, it is believed that autodeposited coatings formed fromexternally stabilized resins comprise resin molecules which are joinedby metal atoms which link together negatively charged hydrophilic groupsof the surfactant molecules which are adsorbed on the surfaces ofdifferent resin molecules. (The source of the metal atom is the metalsubstrate being coated as metal is dissolved therefrom during formationof the coating.) The joining of such resin molecules is based onphysical bonding in that the surfactant is physically adhered to thesurface of the resin particle. In contrast, the use of an internallystabilized resin results in a joining of the resin molecules by chemicalbonding or linking of the resin molecules through the ionizable groupswhich are chemically attached to the resin molecule and to the metalatom, which functions to chemically link ionizable groups of differentresin molecules. In addition, resin molecules are chemically linked tothe metallic substrate through ionizable groups of the resin. As will beseen from examples reported hereinbelow, the adherence to the metalsubstrate of freshly applied coatings of this type is remarkably high,this being attributed to the formation of the aforementioned type ofchemical bonding.

The term "essentially free of chromium", when used herein and in theclaims, means that the autodeposited coating contains little or nochromium, and to the extent that a small amount of chromium is presentin the coating, the source thereof is one other than that deriving fromtreatment of the uncured coating with a chromium compound-containingsolution which is designed to improve the corrosion resistance of thecoating.

EXAMPLES

Examples below are illustrative of the present invention. Comparativeexamples are set forth also.

Example 1-Autodepositing Composition

This example illustrates the formulation of an autodepositingcomposition of this invention. The latex used contains a vinylidenechloride copolymer which is prepared by copolymerization with a watersoluble ionic stabilizer such as sodium sulfoethyl methacrylate. Acomposition was prepared by admixing the following:

    ______________________________________                                                             Amounts                                                  ______________________________________                                        Saran 143 latex        93.0 g                                                 Aquablak S (black pigment dispersion)                                                                3.0 g                                                  hydrofluoric acid      2.3 g                                                  ferric fluoride        3.0 g                                                  deionized water        to make 1 liter.                                       ______________________________________                                    

The Aquablak S dispersion (available from Borden) was thinned with anequal weight of deionized water to produce a consistency approximatelyequal to that of the latex. While stirring continuously, the latex wasslowly added to the diluted black pigment dispersion. The total elapsedtime of mixing to prepare a 1 liter bath is approximately one minute.The mixing time is not critical to the preparation of performance of thebath, but is mentioned here merely to point out that careful andreproducible procedures should always be used in the preparation of acoating composition to assure uniformity from batch to batch. When theblack pigment dispersion has been uniformly blended with the polymerlatex, deionized water is added with continuous stirring. A solutioncomprising the hydrofluoric acid and the ferric fluoride is added to themixture with continuous stirring in such a volume that the blendapproaches 1 liter of volume, for example, 950 ml. Deionized water isthen added to bring the total volume of composition to exactly 1 liter.The resulting composition comprises 5% by weight of polymer coatingsolids.

Example 2-Coating and Property Evaluation

This example illustrates the coating use of the composition of Example 1and some of the properties that are observed for the resulting coatings.

Mild steel test panels, for example, unpolished cold rolled Q-panelswhich are commercially available, are cut to 3-inch by 4-inch size andcleaned in heated alkaline cleaner solution by immersion or sprayapplication or both. The panels are then rinsed with water. The panelsare then immersed in the coating composition of Example 1 for 90seconds. When the panels are removed from the coating composition, theyare rinsed with water and baked for 10 minutes at 100° C.

The coated panels are then scribed and subjected to acceleratedcorrosion testing by exposure to 5% neutral (ASTM B-117) salt spray at95° F. for 500 hours. After testing, the coated panels show negligibleloss of adhesion at the scribe (less than 1 mm) and no failure in anyform on the remainder of the panels.

The coated panels show no failure whatever when exposed to 100% RelativeHumidity at 95° F. for 1000 hours. When subject to 160 inch-lbs ofimpact with a Gardner Coverall Impact Tester using a half inch ball, thecoatings show no loss whatever, even when the impacted area isvigorously tested with masking tape which is applied to the impactedarea and forcefully ripped from the surface. The coatings show the sameresistance to impact and tape after being subjected to a temperature of70° C. for 10 days.

Example 3-Hardness Evaluation

The coatings of Example 2 have a uniform thickness of 0.5 mil (12.7μ).The coatings cannot be scratched by a pencil with a hardness of lessthan 5H-6H. The coatings show very good resistance to solvents. Whensubjected to the Gravelometer test in which gravel of assorted sizes isfired at the coated surface under high air pressure, the coatings rate7-plus (where 0 represents complete failure and 10 represents nofailure). When subjected to Gravelometer followed by 500 hours of saltspray testing (ASTM B-117), the coated panels still rate 6.

This represents an extraordinary advance over presently availablecommercial autodeposited coatings. For example, state of the artcoatings 1.0 mil thick (25.4 microns) can be scratched by any pencilharder than an F which is a very soft pencil. State of the art coatingsare not solvent resistant. Moreover, even when rinsed in hexavalentchromium-containing solutions to improve their corrosion resistance,presently available autodeposited coatings will, when subjected toGravelometer followed by 500 hours of salt spray testing (ASTM B-117),give ratings of 1-2 out of a possible 10. Such low ratings representvirtually total failure, although it should be understood that this is avery severe test method.

Example 4-Solvent Evaluation

This example illustrates the excellent solvent resistance of theautodeposited coatings of Example 2. In the following table, coatings ofthe present invention are compared with commercially availableautodeposited coatings by subjecting them to the action of varioussolvents frequently encountered by automobiles. The conditions used foreach solvent represent the more difficult tests to which automotivemanufacturers subject coatings. The pencil hardness of the coating ismeasured before and after exposure to the solvents with the followingresults.

    ______________________________________                                                       Pencil Hardness After Test                                                      State of the                                                                             Example 2                                         Solvent Soak Test                                                                              art coatings                                                                             coatings                                          ______________________________________                                        None             F          5H-6H                                             Motor Oil                                                                     24 hr Room Temp. B          5H                                                24 hr 180° F.                                                                           HB         5H                                                 2 hr 180° F. plus                                                                      F          5H                                                 2 hr air dry                                                                 Gasoline                                                                       5 hr Room Temp. less than 6B                                                                             5H                                                Ethylene Glycol                                                                2 hr Room Temp. plus                                                                          HB         3H                                                 2 hr air dry                                                                 Brake Fluid                                                                   30 min Room Temp.                                                                              HB         5H                                                16 hr Room Temp. less than 6B                                                                             HB                                                ______________________________________                                    

Example 5-Coating Thickness

This example shows the relationship between the coating thicknessproduced by immersion of mild steel panels into the composition ofExample 1 and the time of immersion.

    ______________________________________                                        Time of Immersion (min)                                                                        Coating Thickness (mils)                                     ______________________________________                                        1.5              0.45                                                         3.0              0.6                                                          5.0              1.0                                                          10.0             1.6                                                          30.0             3.0                                                          ______________________________________                                    

Example 6-Corrosion Resistance

In this example, coatings of various thicknesses were autodeposited byimmersion for various times in the composition of Example 1. Thecoatings were then tested in salt spray for 504 hours (ASTM B-117). Withreference to the test results reported below, it should be appreciatedthat no autodeposited coatings have ever achieved such high corrosionresistance in the prior state of the art without the use of chromaterinses following autodeposition, nor have prior state of the artautodeposited coatings ever achieved such high corrosion resistance whenbaked at such a low temperature (100° C.).

    ______________________________________                                                         Salt Spray Resistance                                        Coating Thickness (mils)                                                                           Scribe*   Field*                                         ______________________________________                                        0.7                     1 mm   9                                              0.6                  1.0-0.5 mm                                                                              9.5                                            0.5                    1-2 mm  9                                              0.4                  1.0-0.5 mm                                                                              9                                              0.35                    2.5 mm 8                                              ______________________________________                                         *The scribe ratings indicate the loss of coating at the scibe in              millimeters when the panels are removed from the salt spray cabinet and       immediately scraped vigorously by holding a spatula with its blade at         90° to the coated surface and scraping the coating back and forth      until all loosely adhering material is removed. The above ratings are all     excellent. The field ratings are based on the number and size of rust         spots over the entire panel with 0 representing total failure and 10          representing no failure whatever. The above ratings are very good.       

Example 7-Additional Evaluations of Corrosion Resistance

In this example, mild steel panels were immersed in a composition asdescribed in Example 1 to produce coatings having a uniform thickness of0.4 mil. The panels were then subjected to salt spray testing (ASTMB-117) for various lengths of time. The panels were then rated for lossof adhesion at the scribe and amounts of corrosion on the remainder ofthe panel. The results are as follows.

    ______________________________________                                                       Salt Spray Performance                                                        (ASTM Ratings*)                                                Hours in Salt Spray                                                                              Scribe  Field                                              ______________________________________                                        336                9       10                                                 504                8.5     10                                                 672                8       9                                                  1152               7.5     7                                                  2112               7.5     8                                                  ______________________________________                                         *A "0" rating indicates total failure, a 10 indicates no failure whatever                                                                              

Example 8-Resistance to Rinsing

In this example, a special method was developed to test the ability offreshly autodeposited, uncured coatings to withstand vigorous rinsing.Deionized water is directed downwardly from a narrow nozzle to impactthe freshly autodeposited coating at a glancing angle. The water leavesthe nozzle with a pressure of 2.5 pounds per square inch and the coatedpanel is located just 6 inches under the nozzle. The panel is held in afixture at an angle of 45° from the liquid stream. If there is no signof redispersion or loss of coating integrity after 1 minute, a sharpmetal object is used to scribe a horizontal line through the wet coatingto bare metal at the area which is impacted by the water to act as a"stress raiser", and the water continues to impact the autodepositedcoating for another two minutes. This test is much more rigorous thanthe relatively less demanding needs of industrial spray rinsing.

Coatings were autodeposited from a composition like Example 1 to producecoatings of the following thicknesses: 0.35; 0.5; 0.7; and 1.0 mil. Thefreshly autodeposited uncured coatings were then subjected to the aboverinsability test. There wasn't any sign of failure either byredispersion or loss of coating integrity at any of the coatingthicknesses tested. This test cannot be consistently passed by prior artfreshly formed autodeposited coatings.

Example 9-Curing in Hot Water

This example describes the rapid, energy efficient method of curingautodeposited coatings of the present invention by hot water. Coatingsof 0.5 mil thickness were formed by immersion of thick-walled hot rolledsteel parts into a composition like that of Example 1. The procedureused was as follows: cleaned in hot alkaline cleaner; rinsed intapwater; autodeposited for 90 seconds; rinsed in tapwater; and immersedfor 5 seconds in water at 185° F. Coating properties such as, forexample, salt spray resistance, were equivalent to those obtained bybaking. After 500 hours of salt spray testing (ASTM B-117), the partsshowed less than 1 mm of adhesion loss at the scribe or a rating of9-plus out of 10, and the remainder of the part was excellent with nosigns of corrosion.

The same parts required 25 minutes in a forced draft oven at 212° F.just to reach curing temperature because the mass of the steel absorbedthe energy before the coating could be brought to the curingtemperature.

Example 10-Curing in Steam

In this example, coatings of 0.5 mil thickness were autodeposited oncold rolled mild steel panels by immersion in a composition like that ofExample 1 for 90 seconds. After removal from the coating bath, thepanels were allowed to stand in air for 60 seconds to permit thesupernatant coating composition to react completely with the metalsurface. The panels were then rinsed in tapwater and placed in a lowpressure steam chamber for curing. In two cases, oven curing was usedwith or without steam curing. The panels were then scribed and placed insalt spray for 168 hours and 336 hours. The following table lists theresults.

    ______________________________________                                                     Salt Spray Performance                                           Steam Cure                                                                             Oven Cure Scribe       Field                                         Time     Time      168 hr   336 hr                                                                              168 hr 336 hr                               ______________________________________                                        30 sec   --        8        7     9      9                                    2 min    --        8        7     9      9                                    5 min    --        8        7     8      5                                    10 min   --        0        0     0      1                                    --       10 min    8        8     9      9                                    2 min     2 min     7+      7     9      9                                    ______________________________________                                    

Example 11-Curing at Various Temperatures

In this example, cold rolled mild steel panels were immersed in acomposition like Example 1 for 90 seconds to deposit 0.9 mil of coating.The coating is very uniform and has near specular gloss. Curing of thecoating can be effected at very low temperatures, for example, from roomtemperature to about 120° C. The following table indicates the excellentcorrosion resistance which can be obtained with autodeposited coatingsof this composition when cured at various temperatures.

    ______________________________________                                                        Salt Spray Performance (168 hrs)                              Curing Temperature, °C.                                                                ASTM B-117 Rating*                                            ______________________________________                                        20                 8 plus                                                     40              7                                                             50              8                                                             60                8.5                                                         75              7                                                             90              8                                                             105             9                                                             120             9                                                             ______________________________________                                         *The ratings shown are for scribe only. Field performance was 10 in every     case.                                                                    

The salt spray results shown are excellent even though the curingtemperatures used were as much as 140 to 180 centigrade degrees coolerthan are possible with prior art autodeposited coatings.

The next group of examples is based on the use of vinylidene chloridecopolymer latexes which are externally stabilized by anionic surfactantsadsorbed to the resin particles.

Example 12-Autodepositing Composition

The following acidic aqueous coating composition was prepared bycombining

    ______________________________________                                                            Amounts                                                   ______________________________________                                        latex containing about 60 wt. % solids                                                              167 g                                                   black pigment dispersion                                                                             5 g                                                    ferric fluoride        3 g                                                    hydrofluoric acid      2.3 g                                                  water                 to make 1 liter                                         ______________________________________                                    

The resin of the latex used in the above composition comprises acopolymer of vinylidene chloride, vinyl chloride, ethylhexyl acrylate ormethacrylate, and acrylic acid. The vinylidene chloride content may varyfrom 60 to 76 weight %. The ethylhexyl acrylate or methacrylate may varyfrom 4-10 weight %. The acrylic acid may vary from 1 to 4 weight %. Thelatex is externally stabilized by the adsorption of sodium laurylsulfate on the resin particles. The black pigment dispersion comprises afine particle furnace black dispersed in water by an anionic surfactant.The pigment content is 45 weight % of the dispersion. The latex has aminimum film formation temperature (MFT) of 12°-15° C. A film cast fromthe above latex by conventional methods such as drawdown or immersionhas a very low permeability (20 g/meter² /25 microns/day).

Example 13-Coating and Property Evaluation

This example shows the physical properties of coatings formed byautodeposition from the composition of Example 12. Coatings wereautodeposited to a film thickness of 0.9 mil in 90 seconds. After curingat 90° C. for 10 minutes, the coatings showed near specular gloss withreadings of 90 to 95% reflectivity at 60° using a Gardner Colorguardmeter. The coatings were impacted with a Gardner Coverall impact testerusing a half-inch diameter ball at 160 inch-pounds. When the coating atthe impacted area was tested by applying Scotch brand tape to thesurface and then rapidly pulling it away, there was no sign of adhesivefailure either by reverse or direct impact, that is, neither the concavenor the convex side of the impacted area showed any loss of adhesion. Instill another physical test, the panels were bent back on themselves180° and the bend was firmly pressed in a vise. This is sometimes calleda zero-T bend. Scotch brand tape was placed in good contact with thebend and then rapidly pulled away without the slightest sign of failureof the coating. When the coating hardness was tested with pencils ofvarying hardness, it was found that even when the coating is air-dried,the coating cannot be scratched by anything softer than a 3H pencil.

Example 14 - Autodepositing Composition

This example illustrates the formulation and deposition of high gloss,colored coatings based on the type of latex of Example 12.

    ______________________________________                                                           Amounts                                                    ______________________________________                                        Haloflex 20 latex    180 g                                                    Dowfax 2A1 surfactant                                                                               0.6 g                                                   Sup-R-Conc L Brilliant Red 2R                                                                       10 g                                                    pigment dispersion                                                            ferric fluoride       3 g                                                     hydrofluoric acid     2.3 g                                                   water                to make 1 liter                                          ______________________________________                                    

The following procedure is used to prepare the above composition. Adjustthe pH of the latex to 7.1 by addition of 2% ammonia solution. In aseparate container, dilute the pigment with just enough deionized waterto induce the paste to flow. Stir in the Dowfax 2A1 surfactant (sodiumsalt of an alkylated diphenyl oxide disulfenate). While agitating thelatex, slowly stir in the pigment dispersion. When the color is uniform,add the water. Finally, stir in an aqueous solution containing theferric fluoride and the hydrofluoric acid.

Immerse clean, mild steel panels in the above composition for 3 minutesto deposit 1.0 mil of coating. Cure the coating for 10 minutes at 100°C. The cured coatings are uniform, bright red, with near specular gloss.

Example 15 - Corrosion Resistance

This example illustrates the high corrosion resistance of autodepositedcoatings formed from a composition like that of Example 11. Clean, mildsteel panels were immersed in the coating composition for 90 seconds.They were then baked at 90° C. for 15 minutes. Salt spray performanceafter various exposure times is shown in the following table.

    ______________________________________                                        Exposure Time Salt Spray Performance                                          (ASTM B-117)  Ratings                                                         ______________________________________                                        648 hours     8                                                               840 hours     8                                                               1008 hours    7                                                               ______________________________________                                    

This performance is superior to that of autodeposited coatings formedfrom prior art compositions. Treatment with chromium solutions were notrequired to obtain this performance and the curing temperature used was70 Centigrade degrees cooler than is used by current commercialautodepositing finishing lines.

Example 16 - Autodepositing Composition and Corrosion Resistance

This is an example of a latex in which the poly (vinylidene chloride)particles are externally stabilized by the adsorption of surfactant onthe surface of the particles. The latex has a high vinylidene chloridecontent which is reflected in the density of the latex which is 10.43lbs per gal. The low concentration of surfactant in the aqueous phase orserum is shown by the high surface tension, 52 dynes/cm.

    ______________________________________                                                      Amounts                                                         ______________________________________                                        Serfene 120 latex                                                                             200 g                                                         ferric fluoride  3 g                                                          hydrofluoric acid                                                                              2.3 g                                                        water           to make 1 liter                                               ______________________________________                                    

Cold rolled, mild steel panels were treated as follows: cleaned in hotalkaline solution; rinsed with water; immersed in the aboveautodepositing composition for 90 seconds; allowed to stand in air fortwo minutes; rinsed with water; and baked for 10 minutes at 120° C. Thecured coatings, which were 1.25 mils thick, were subjected to salt spraytesting for various periods of time, as reported in the following table.

    ______________________________________                                                       Salt Spray Peformance                                          Time in Salt Spray                                                                           ASTM B-117 Ratings                                             ______________________________________                                        168 hours      7                                                              336 hours      6                                                              504 hours      6                                                              672 hours      6                                                              840 hours      6                                                              ______________________________________                                    

This is excellent performance despite the fact that nochromium-containing solutions were used to improve the corrosionresistance and the curing temperature was 40° to 80° Centigrade degreeslower than that used in curing prior art autodeposited coatings.

The next group of examples illustrates the preparation of variouslatexes which include particles of resin comprising copolymers preparedfrom vinylidene chloride and other monomers by emulsion polymerizationreactions which include relatively small amounts of emulsifier. Thisgroup of examples illustrates also the preparation of autodepositingcompositions containing the latexes and the evaluation of autodepositedcoatings formed therefrom.

Examples 17 to 25-Preparation of Latexes and Evaluation ofAutodepositing Compositions Including the Same

There were placed in a reaction vessel 53.76 kg of demineralized waterand 0.005 kg of ferrous sulfate.7H₂ O. The contents of the vessel wereheated to about 30° C. and rendered oxygen-free by twice evacuating andpurging with nitrogen. Thereafter 0.157 kg of emulsifier (100% sodiumlauryl sulfate) was added. The following monomeric constituents wereplaced into a separate container: 34 kg vinylidene chloride; 0.6 kgacrylic acid; 3.4 kg methylacrylate; and 2 kg acrylonitrile. Eight kg ofthis 40 kg monomeric mixture were then placed in the reaction vessel.After 10 minutes of agitation, each of the following initiator solutionswas added to the reaction vessel: (A) 0.3 kg of a solution consisting of0.025 kg sodium disulfite in 2 kg of demineralized water; and (B) 0.62kg of a solution of 0.05 kg ammonium peroxydisulfate in 4 kg ofdemineralized water. As a result of the heat of reaction, thetemperature of the reaction mixture increased to about 35° C. At the endof the heat release, the remaining 32 kg of the monomeric mixture andthe remainder of each of the aforementioned solutions were added to thereaction vessel. The monomeric mixture was added over about a 2-hourperiod, whereas the addition of each of the initiator solutions wascompleted after about 50 minutes. The addition of the monomeric mixtureand initiator solutions was controlled in a manner such that thereaction temperature did not increase above about 37° C. Following theend of the exotherm, and after no further refluxing, the reactiontemperature was raised to 50° C. and maintained for about 30 minutes. Toreduce residual monomer content, the product of reaction was distilledbriefly at about 15° C. under reduced pressure. The 40% emulsion thusobtained contained less than 1 wt. % coagulate.

The above basic reaction process was utilized to polymerize othermonomeric mixtures, as identified in Table 1 below, Example No. 17 beingthe monomeric mixture described above.

                                      TABLE 1                                     __________________________________________________________________________    Wt. % Based on Total Wt. of Monomer                                           Ex.                                                                              Vinylidene                                                                          Acrylic                                                                            Methyl                                                                             Ethyl                                                                              Butyl                                                 No.                                                                              chloride                                                                            acid acrylate                                                                           acrylate                                                                           acrylate                                                                           Acrylonitrile                                                                        MMA*                                      __________________________________________________________________________    17 85    1.5  8.5  --   --   5      --                                        18 90    2    8    --   --   --     --                                        19 90    2.5  --   --   7.5  --     --                                        20 85    2.5  --   --   --     6.2  6.3                                       21 90    1.5  8.5  --   --   --     --                                        22 81    1.5  --   --   8.5  9      --                                        23 80    1.5  --   --   13.5 5      --                                        24 81    1.5  --   8.5  --   9      --                                        25 85    1.5  --   --   8.5  5      --                                        __________________________________________________________________________     *Methyl methacrylate                                                     

Autodepositing compositions were prepared from each of the latexes ofTable 1 as follows. Two hundred grams of latex were diluted with 400 mlof distilled water. Fifty ml of activator solution composed of water, 15ml of a 21 percent solution of hydrofluoric acid and 4.1 grams of ferricfluoride were diluted with about 100 ml of water and then slowly pouredinto the latex mixture. Distilled water was then added to make the finalvolume of the coating bath 1 liter. Steel panels were autophoreticallycoated therein, and the coatings were cured for 5 min at 100° C. withoutprevious rinsing in a chromate solution. The coated panels weresubjected to a salt spray test according to ASTM 117-73 for 120 hours. Adiagonal cross (St. Andrew's cross) was scratched into the coating. Theevaluation standards are as follows: (A) 1=no corrosion; (B) 2=slightcorrosion along the scratch; (C) 3=corrosion also outside of thescratched area, beginning from the cross, and with cracking of thecoating; (D) 4=marked under-rusting, dissolution of the coating. Theresults of the test are set forth in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Ex. No.       Evaluation                                                      ______________________________________                                        17            1-2                                                             18            1-2                                                             19            2                                                               20            2                                                               21            2                                                               22            1-2                                                             23            1-2                                                             24            1-2                                                             25            1-2                                                             ______________________________________                                    

The above examples illustrate well important advantages which flow fromuse of the present invention. It should be understood that the presentinvention can be utilized in connection with autodeposited coatingsformed on metallic surfaces other than the ferriferous surfaces whichare exemplified hereinabove. Accordingly, the invention can be usedwidely in forming coatings on various types of metallic objects whichcan be used in an almost unlimited number of applications.

AMPLIFICATION OF THE DISCLOSURE

The foregoing disclosure may be amplified by the following furtherembodiments, which define essentially the same invention in slightlydifferent terms and from a slightly different perspective, and whichalso set forth slightly variant parameters.

An amplified embodiment of this invention comprises polyvinylidenechloride dispersions for autodeposition dip lacquering in acid baths,containing the following, dispersed in water:

a copolymer comprising 70-95 wt % vinylidene chloride and additionalmonomers,

polymerization emulsifiers, and

customary auxiliaries, characterized in that the copolymer, in additionto other comonomers, contains at least one acid comonomer with --COOHand/or --SO₃ H groups, and the concentration of the polymer emulsifier,based on the total dispersion, amounts to 0.01-03 wt %.

An additional embodiment is a process for the production of stabilizedvinylidene chloride dispersions, characterized in that under theconditions customarily employed in the production of polyvinylidenechloride dispersions first 5-25% by weight of the total monomer mixtureare subjected to emulsion polymerization in the entire volume of waterand emulsifier, and then the remaining quantity of monomer and, ifdesired, additional polymerization initiator are added approximately inproportion to the progress of the polymerization reaction.

In one embodiment, the stabilized polyvinylidene chloride latexdispersions differ from the prior art in that a specific, extremely lowemulsifier concentration is used in preparing them; the emulsifierconcentration, relative to the total dispersion, preferably amounting to0.3-0.01% by weight. Dispersions with more than 50% by weight of polymercan be produced with such low emulsifier concentrations. It is alsodesireable in this embodiment to adjust the solids content of thepolyvinylidene chloride dispersion to 25-40, preferably 30-40, % byweight.

The preferred emulsifier for the production of the stabilized latexdispersions of this embodiment is lauryl sulfate. Lauryl sulfate isdefined here as also comprising sulfated coconut fatty alcohol orsulfated distillates of coconut fatty alcohol. Additional suitableemulsifiers are the sulfonation products of lauryl alcohol reacted with2 to 5 mols of ethylene oxide or the corresponding coconut alcohols.Also suitable as emulsifiers are sulfated alkyl phenol ethers, e.g., asulfated reaction product of nonyl phenol with 2-5 mols of ethyleneoxide. Finally alkyl benzene sulfonate is a suitable polymerizationemulsifier. The polymerization emulsifiers mentioned may be usedindividually, in mixtures, or as salts, e.g., sodium or ammonium salts.

The polyvinylidene chloride dispersions in accordance with thisembodiment of the invention contain at least one copolymer with acidgroups polymerized into the polymer composition. In this embodiment'sprocess comonomers with the carboxyl groups or with sulfonic acid groupsor monoesters of sulfuric acid may be used. The acid group-containingmonomers are used in quantities of 0.5-4, preferably 1-2, % by weightbased on the weight of the copolymer. Acrylic acid is preferred as theacid group-containing monomer because of its good copolymerizabilitywith vinylidene chloride. However it is also possible to use methacrylicacid, itaconic acid, hemi-esters of maleic acid with C₁₋₄ alcohols,hemi-esters of fumaric acid, or crotonic acid. Additional suitable acidgroup-containing monomers are: vinyl sulfonic acid,2-acrylamido-2-methyl propane sulfonic acid or 2-acryloyl- or2-methacryloyl-ethane-sulfonic acid or -propane sulfonic acid. All ofthe acids mentioned can be used as such or in the form of their lithium,sodium, potassium or ammonium salts.

The polymer on which the stabilized polyvinylidene chloride latexdispersion in accordance with this embodiment of the invention is basedalso consists of additional comonomer building blocks. Thusacrylonitrile may also be used in a preferred embodiment of theinvention. The quantity of acrylonitrile used may be up to about 25,preferably 1-8, most preferably 3-6, % by weight based on the weight ofthe copolymer.

In accordance with a further embodiment of the invention vinyl chloridecan be used. In addition esters of acrylic acid or methacrylic acid aresuitable as comonomers. Thus the esters of these acids with methanol,ethanol, propanol, isopropanol, butanol, ethylhexanol, e.g.,2-ethylhexanol, or hexanol can be used. Additional suitable esters ofacrylic or methacrylic acid include, for example, 2-hydroxyethylacrylate or methacrylate or 2-hydroxypropyl acrylate. In addition tothese compounds, acrylamide and substituted acrylamides, for exampleN-butyl-acrylamide, or in general N-alkylacrylamides with 2-4 C atoms inthe alkyl substituent may be used.

In any case, the composition of the polymers on which the stabilizedlatex dispersions in accordance with this embodiment of the invention isbased is to be selected such that the vinylidene chloride content is70-95, preferably 80-90, % by weight based on the total polymer weight.

The preparation of the stabilized polyvinylidene chloride latexdispersions in accordance with this embodiment of invention ispreferably performed using a modified monomer inflow process. Thisprocess provides that the entire quantity of water and the quantity ofemulsifier be placed in advance under nitrogen in a reaction vessel andemulsion polymerization started with part of the initiator plus 5-30,preferably 10-20, % by weight of the total quantity of monomer. Thisemulsion polymerization is continued up to a conversion rate of greaterthan 80%. After this the residual monomer quantity is added continuouslyor batchwise. It is preferred to adapt the addition rate to the courseof the reaction, i.e., in particular to add it as rapidly, as themonomers are copolymerized. In the process in accordance with theinvention the monomers can be added as mixtures or, if necessary on thebasis of the copolymerization parameters, separately and at differentaddition rates. Other details of this type of emulsion copolymerizationare generally known in the art.

Polyvinylidene chloride latex dispersions produced in accordance withthis embodiment of the process of the invention are essentially freefrom undesireable coagulate. This is astonishing, since extremely smallquantities of emulsifier are used in comparison to known processes inthe art.

Initiators customarily used in the emulsion polymerization of vinylidenechloride can be employed in the production process in accordance withembodiment of the invention. For example, redox initiators on the basisof potassium or ammonium peroxy-disulfate/sodium sulfite may beemployed. In addition organic peroxides, e.g., cumene hydroperoxide, orwater-soluble azo compounds may be used. To produce autodepositionbaths, the polymer dispersions in accordance with this embodiment of theinvention are substituted in equal quantities (based on the polymer) forknown latex dispersions. Specifically the dispersions are first dilutedwith water until a polymer content of 3-15, preferably 5-10, % by weightbecomes established after addition of all other components. Then ifdesired a pigment such as carbon black in dispersion is added and mixedwith a customary starter solution. Such starter solutions may contain,for example, hydrofluoric acid, its iron salts, and oxidizing agentssuch as H₂ O₂. Measurement of the redox potential is generally performedto test the baths. An advantageous value for the redox potential isbetween -300 and -400 m.v.

To obtain the autodeposition coating according to this embodiment, ametal sheet cleaned according to the usual methods, made of zinc,aluminum or preferably steel, is immersed in the bath for 20 to 200 sec.The layer thickness achieved depends on the immersion time. The wetfilms thus produced may be cured in the usual manner. In general a heattreatment cure at temperatures of approximately 80° C. is performed.

COMPARISON EXAMPLES C-1 and C-2

Examples 18 and 19 were repeated except that instead of 0.157 kg oflauryl sulfate emulsifer, 1.00 kg was used, and the volume of water wascorrespondingly reduced. The result was a non-stabilized (i.e., priorart) polyvinylidene chloride latex, as is demonstrated by the following.

Autodeposition baths were prepared in the same manner as Examples 18 and19, only with the products of Example C-1 and C-2, and tested on steelsheets in the identical manner. The results were as follows:

                  TABLE 3                                                         ______________________________________                                        Example No.    Evaluation                                                     ______________________________________                                        C-1            3                                                              C-2            3-4                                                            ______________________________________                                    

Comparing Tables 2 and 3, it is clear that the maximum amount ofemulsifier is critical, which would not be expected from the prior art.These results are in accordance with the comments regarding criticalmicelle concentration, infra.

What is claimed is:
 1. An autodepositing composition comprising anacidic aqueous solution of activator consisting essentially ofhydrofluoric acid in an amount sufficient to impart to the composition apH within the range of about 1.6 to about 5.0 and a soluble ferriciron-containing compound in an amount equivalent to about 0.025 to about3.5 g/l of ferric iron and having dispersed therein resin solids of aninternally stabilized copolymer comprising: (A) at least about 80 wt. %of vinylidene chloride; (B) one or more monomers selected from the groupconsisting of methacrylic acid, acrylic acid, methyl methacrylate,methyl acrylate, ethyl acrylate, butyl acrylate, acrylonitrile,methacrylonitrile, vinyl chloride, acrylamide and methacrylamide; and(C) a water-soluble ionic material which includes an inorganic ionizablegroup and which has the formula:

    R--Z--Q--SO.sub.3 M.sup.+

wherein R is vinyl or alkyl substituted vinyl, Z is a difunctionallinking group, Q is a divalent hydrocarbon group, and M is a cation. 2.A composition according to claim 1 wherein said water-soluble ionicmaterial is sulfoethylmethacrylate.
 3. A composition according to claim1 wherein said monomer is selected from the group consisting ofmethacrylic acid, methyl methacrylate, acrylonitrile, and vinylchloride.
 4. A composition according to claim 1, wherein said solubleferric iron-containing compound is present in an amount equivalent toabout 0.3 to about 1.6 g/l of ferric iron.
 5. A composition according toclaim 4 wherein the source of the soluble ferric iron-containingcompound is ferric fluoride.
 6. A composition according to claim 1,comprising about 3 to about 10% by weight of said internally stabilizedcopolymer.
 7. A composition according to claim 4 comprising about 3 toabout 10% by weight of said internally stabilized copolymer.
 8. Acomposition according to claim 5 comprising about 3 to about 10% byweight of said internally stabilized copolymer.